The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast for high intensity discharge lamps that accommodates lamp replacement and ignition while power is applied to the ballast.
Electronic ballasts for powering high-intensity discharge (HID) lamps usually have a timed ignition period of about 20 to 30 minutes, during which time high voltage pulses (e.g., of 3000 volts or more) are provided in order to ignite the lamp. For reasons of safety and reliability, if the lamp does not ignite within the timed ignition period, the ballast stops providing the ignition pulses.
When a HID lamp reaches the end of its useful operating life, the lamp either self-extinguishes or is extinguished by end-of-life protection circuitry in the ballast. If the lamp is replaced, the fact that a new lamp has been installed is not easily sensed because, unlike fluorescent lamps, HID lamps do not have filaments. Thus, in order to restart the timed ignition period and provide high voltage pulses for igniting the lamp, the input power to the ballast needs to be toggled in order to reset the ignition timer circuitry within the ballast. While toggling of the input power will work fine in applications where only one ballast is coupled to an electrical branch circuit, it is not a practical solution in typical installations where multiple ballasts and lamps are powered from the same branch circuit. In the latter case, toggling of the input power will extinguish the other operating lamps, and the hot reignition of the extinguished lamps may take up to 10 minutes or more.
Currently, no manufacturer appears to offer an electronic HID ballast with a live relamping feature. Moreover, although there are a number of possible approaches that may be used to address this problem, each has significant disadvantages.
In one possible approach, the ballast may be designed such that ignition pulses are turned on and off at a predetermined rate. For example, after the lamp has extinguished (due to end-of-life), the ignition pulses could be turned on (e.g., for a few minutes) and turned off (e.g., for a few hours). This will ensure that a replaced lamp will eventually light up when the ignition pulses are turned on. The disadvantages of this approach include safety and reliability concerns due to the periodic (though not continuous) application of ignition pulses, as well as the fact that ignition of the replaced lamp may not occur for as long as several hours following replacement. With regard to the latter disadvantage, it should be appreciated that prompt ignition of a replaced lamp is highly desirable.
Another possible approach involves providing a mechanical switch or reset button on the ballast, wherein the switch or button is momentarily depressed by a user (i.e., the person who changed the lamp) following lamp replacement in order to direct the ballast to provide ignition pulses. An obvious disadvantage of this approach is that the switch or button would have to be made accessible to the user, which is not practical in most installations.
A third approach would be to equip the ballast with a wireless receiver, wherein a user may send a signal to the ballast via a wireless remote control in order to direct the ballast to provide ignition pulses following lamp replacement. Because of the additional circuitry that would be required, this approach is quite complex and expensive.
Thus, a need exists for an electronic HID ballast that accommodates live relamping in an efficient and cost-effective manner, and without negatively affecting other ballasts and lamps that are operating at the same time. Such a ballast would represent a considerable advance over the prior art.